Sounds are characterised by their frequency and vary from the infrasonic vibrations (typically <20 Hz) of earthquakes and volcanoes to the ultrasonic (typically >100 kHz) calls of dolphins and bats.
Experiments have shown that a healthy young person hears all sound frequencies from approximately 20 Hz to 20 kHz. However, a person's lifestyle and age can reduce the sensitivity of the persons hearing, effectively reducing the audible frequency range. Elderly persons for example, typically cannot hear the upper range of frequencies. Accordingly, it is possible to target generally younger persons with sound waves which have a frequency in the upper quarter of the audible range, i.e. above 15 kHz, since the majority of older persons will be incapable of detecting the sound.
The present invention is concerned with the generation of sound waves that can be heard by a subset of a species of individuals. In particular, the invention deals with the generation of sound waves which can lead to the dispersing of individuals and in particular groups of young persons in places where their congregation is unsuitable, such as outside shops and street corners. It is common for large numbers of young persons to gather at certain locations, which can jeopardise their own safety and which can often cause a hindrance to other members of the public. In addition, groups of rowdy young people can sometimes intimidate other passers-by.
However, conventional methods of dispersing crowds of young people can often be disruptive to other members of the public and can be confrontational, particularly if the police are required.
It would therefore be desirable to provide improved methods, devices and systems for dispersing crowds, particularly with preferential effects on young persons.